A Physics Party Trick that Sucks… Liquid

Well, this is going to be sweet, short and to the point: Fire in closed spaces can really suck.

Ha, I was dying to use that pun for a while now, and here I had the chance. This experiment is a really short and sweet one, and can join your mental arsenal of “party tricks” for the partying geeks. It can really impress anyone, and from now on – you are going to know what makes this happen.


Notice: This experiment is incomplete, and unclear. There were several attempts to correctly state the situation, but at the moment, a new re-make is planned to explain exactly and thoroughly what is happening to create this phenomenon.

Air is a fascinating thing, but sometimes it can be an obstacle. We will see that in future experiments, where the existence of air (or, more precisely, of oxygen) can hinder an experiment and render it unexperimentable. … Right. I think I need a dictionary replacement.


In case this isn’t completely clear, I am going to point out that since we are dealing with a live and exposed flame, the use of any high-percentage alcohol is absolutely not recommended.

I hope that is obvious, but in case it’s not – ALCOHOL IS FLAMMABLE. SO IS GASOLINE. Don’t do something very stupid, don’t use flammable liquids in this experiment!

(Thanks to RedShiftScience for pointing out people might not find this obvious.)


Before I go on to corrections, let me say a word about getting things wrong: Human beings are usually emotional entities, and as such, we tend to take things personally. Science is supposed to be empirical, void from emotions. How do you connect the two? Using the scientific method.

There is no shame in getting things wrong. We are only humans.

The best thing about science and experimentation is to have other people think about things, analyze them, and criticize your work. I not only enjoy that, I think it’s a necessary part of science.

In my video, I explained a few things incompletely, and some even seemed to have come across bluntly wrong (aaa! matter is not created out of nothing, and it does not disappear! in failing to mention that, I sounded like this experiment defies the laws of thermodynamics!). So, I am hereby correcting, adding and subtracting to what I said. I tried to do that well in this post — and then Shane Killian — who noticed this error first – posted a video reply.

So now I can just post it here instead of doing it all over again. Cheers, Shane, GREAT job, and thanks a lot for the correction!


Don’t ever be afraid to try just because you’re afraid to make a mistake.

What is a Vacuum?

A vacuum is a volume of space with no matter in it, and a zero atmospheric pressure. That is the formal definition. That said, there are no places in nature that have absolute vacuum.

We tend to call “Outer Space” a vacuum, but in reality, it is filled with particles, which makes it have some sort of matter, which means it’s not a complete vacuum. But it’s close enough.

Since a vacuum is supposed to have 0 atmospheric pressure (or as close as possible), it “sucks” into it anything that has a different – and higher – pressure. This is due to the tendency to have a balance of pressures — different pressures will try to balance one another, so the lower pressure environment will “suck” matter from the higher pressure environment until both environment are at a balance.

That’s why you see people get sucked out of the airlock in sci-fi movies. It’s one of those things movies got right.

In our experiment, we lowered the pressure and created a semi-vacuum inside the glass, and in turn, it sucked up the liquid around it. Or, more specifically –

What’s going on here?

With this cool little party trick, we are creating a “semi” vacuum inside the clear glass by consuming the oxygen inside it. When the fire consumes the oxygen molecules, it “vacates” a place for – well – whatever else. The pressure inside the glass rises, and since it isn’t sealed, it sucks whatever it is standing on

CORRECTION: The pressure inside the glass increases as the fire heats up the molecules. Oxygen is being “consumed” by the fire, that produces Carbon Dioxide (the matter itself remains, no matter is mysteriously ‘vanishing’ or ‘created’ out of nothing!). But now, the pressures are different and therefore the water outside the glass are pushed inwards — the lower pressure of the INSIDE ‘sucks in’ the liquid around it under the pressure stabilizes.

Thanks to Shane Killian for the correction.

If I were to use a jar and sealed it well while the candle inside fed on the oxygen, the cap would have been “sucked” into the jar mouth, and the jar would have been sealed. Since I am not using a cap, but rather putting the glass on top of liquid (that can “pass through” the edge of the cup), the liquid is sucked inside the glass and stays there, until I release the pressure and allow air in.

This is a really sweet, cool and short experiment, but the best thing about it is that it will help us produce home-style vacuum setting for other experiments. And so, it’s good to know.

Plus, it’s fun. And edible. Woo hoo.

Practical Applications

  • First, this is a cool and easy way of creating home-made semi-vacuum setting, for whatever other experiment you will need. We’ll use this in the future.
  • Here’s a cool practical trick to preserve food for you to consider, though it isn’t precisely the same method, it uses a similar point: If you cook something and wish to save some for later in sealed jars, the best way of doing that is seal the jar while the food is still hot. Once sealed, whatever air inside the jar is trapped, and when the food cools, the air compresses and tightens the jar cap so that it is relatively sealed from the outside. Food will last longer this way, but you will have a bit of a harder time opening the jar.
  • Impress people in parties, collect on bets, and dazzle your dates. What else do you want?



hai i need to know the flashing point of air. at what temperature and pressure the air gets fire. it will get fire?

Name (Required)
Name (Required)

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Fabian Ropiski
Fabian Ropiski

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bachelor party strippers

Physics is really amazing. The video is so great. Thank you for sharing. I agree that the best thing about science and experimentation is to have other people think about things, analyze them, and criticize your work.

john michael balaba
john michael balaba


i want new physics tricks

4 my project

im tired to experiment

i am not a true scientist!!!!!!!!!!!!!

Nick Chira
Nick Chira

Generally speaking, the first video was correct, and the 2nd video is not. In a hydrocarbon combustion reactions, more molecules of oxygen are consumed than molecules of carbon dioxide that are produced, thus dramatically lowering the pressure. As others have noted, the remaining oxygen usually forms liquid water. The reduction in overall air molecules is the dominant force of this physics trick.

As a matter of fact, the change in temperature does have an effect, but it is opposite to the observed overall effect. That is, as you enclose the flame with the glass, the air in the glass heats up, which increases the pressure, thus forcing liquid OUT of the glass. It is then true that the air cools down substantially when the flame goes out, but it is still hotter than the air was previously, and is thus always higher pressure than the surrounding atmosphere, so it still acts to force liquid out, but when the candle goes out the temperature effect can no longer appreciably affect the effect from the number of air molecules in the enclosed space.

It should further be noted that temperature will never affect the overall density of a closed system in any way, shape, or form. As a matter of fact, hot air is generally less dense than cold air as they have more pressure for a given number of molecules.


I have a problem with the statement: "Oxygen is being “consumed” by the fire, that produces Carbon Dioxide "

Fire is heat and light energy produced by the chemical reaction that "consumes" the reactant O2 while also producing the product CO2 (among other things).

I don't think this is trivial because fire is not a tangible thing. It's a name for the combination of types of energy our sense organs can detect during this exothermic reaction.


Heat from the candle flame also displaces air around it. When it goes off, low pressure is created in addition to the explanations offered above.

james yeatman
james yeatman

Hey. im doin a project and i was wondiering if hypotonic transportation would be simular to this expanation . the project im doing is just to demonstrate hypotonic transportation.?


hi, i have just discovered your blog today, nice job, good idea to propose these home experiments but i still not confortable with the explanation. So, I reppeat the experiment two, tree times and after observing some small drops inside the glass, finally i can propose to you another explanation:

candles are made of hydrocarbon, it means atoms of Carbon and atoms of Hydrogen. When the candle are burning C atoms react with O2 molecules to give CO2. So, starting from one molecule of O2, you create another molecule (CO2) , the number of gas molecules do not change (same pressure). But in the same time, the H atoms react with some part of O2 to create H2O (water). This water do not remain as a gas but as a liquid. If you take care, you can see small drops at the internal surface of the glass. So finally a part of O2 gas desepears to produce liquid water and then the pressure decrease inside the glass. Your first explanation was not totally wrong. (sorry for my basic english)

Neil Pharr
Neil Pharr

"Yes, and only Evolution does that! (Kidding!)"

Very interesting comment Tom (no kidding!)

I remember reading in Readers Digest in about 1963, "Life is an inherent property of matter". That got my attention.

Could there be a waining of some cosmic binding force on matter that is not only reflected in the generation of life/mind, but a directional flow in bio-evolution toward more suble expression of mind and thus the appearance of (defies the laws of thermodynamics!”) as your comment?

neil in atlanta


I'd say 'density' (or whatever the correct English word is) explains it best, since CO2 weighs more per volume unit than O2. So it's the volume that's consumed, not the O2 that's still there, but now connected to the C Atom.

Hmm, do I hear another experiment coming? ;-)

Tom S. Fox
Tom S. Fox

"in failing to mention that, I sounded like this experiment defies the laws of thermodynamics!"

Yes, and only Evolution does that! (Kidding!)


Hm, yes, thank you for the correction. You're right, I didn't quite mean it this way, but I sound as if I do. So -- I am fixing the post right now. The fire isn't consuming the molecules! I posted a correction both here and in the video page.

The effect is still about pressure, but matter is definitely not "vanishing" and is not 'created out of nothing'. Of course.


Shane Killian
Shane Killian

Moo, I love you, you're great, but you've got this wrong. Remember that matter cannot be created or destroyed, only change its form. The oxygen atoms are still there; they aren't being teleported off to Loompaland or anywhere. What's happening is that the oxygen is now part of other molecules; for example, oxygen combines with the carbon from the candle to make CO2. Oxygen gas is simply replaced with carbon dioxide (and other gases).

The reason why the milk is being sucked into the glass is because the candle has been heating the air around it. As it consumes the oxygen in the glass, it burns less hot, and eventually burns out entirely. As this happens, the density of the air inside the glass decreases, and this pulls in the milk (or, specifically, this is what causes the greater pressure in the air outside the glass to push in the milk).


When I read about this trick, my theory was like Nick's: that oxygen is converted to much denser CO2. This is because I remember physists in 17th century studying composition of air that way. However, I saw it demonstrated yesterday and water rose good way above 1/3 of the glass. But oxygen only takes 21% of air, so clearly its consumption cannot drive this trick alone.


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